1. Field of the Invention
The present invention relates to decrease in the resistance of an upper transparent electrode of a self-emitting organic electroluminescent device.
2. Description of the Related Art
A self-emitting organic electroluminescent element (hereinafter referred to as an “organic light emitting element”) is expected as an illumination device for a thin-screen display device and a liquid crystal display device.
An organic light emitting display device includes a plurality of organic light emitting elements constituting pixels on a substrate and a driving layer for driving the organic light emitting elements. The organic light emitting element has a structure in which a plurality of organic layers are put between a lower electrode and an upper transparent electrode. The plurality of organic layers include at least a transport layer for transporting electron holes, a transport layer for transporting electrons and a light emitting layer for re-combining the electron holes and the electrons. The holes and electrons injected from electrodes are re-combined in the light emitting layer to emit light by application of a voltage between both of the electrodes.
In the general structure, the transport layer is formed over the entire display panel region and used in common as a transport layer for a plurality of organic light emitting elements. With the constitution described above, only the light emitting layer requires patterning comparable with that of a pixel size. For the patterning comparable with that of the pixel size, a precision mask is generally used. Since the precision mask involves a problem of lowering the mass productivity due to mask exchange, etc., it is preferred to decrease the number of its use.
In a usual organic light emitting display device, the upper transparent electrode of the organic light emitting element is used as a common electrode. Accordingly, when a plurality of organic light emitting elements emit light, the entire current for the display panel flows in the upper transparent electrode. In the case of using a highly-resistive transparent conductive film for the upper transparent electrode, unevenness in applied voltages occurs between a pixel comprising an organic light emitting element at the periphery of the display panel near the power source and a pixel comprising an organic light emitting element at the central portion of the display panel due to the wiring resistance caused of the upper transparent electrode, which, as a result, generates unevenness of luminance.
JP-A-2004-207217 discloses the constitution of an organic light emitting display device using an upper transparent electrode. Auxiliary wiring is formed in a layer level with that of a lower electrode of an organic light emitting element. The auxiliary wiring and the upper transparent electrode are connected in a pixel region to decrease unevenness of the wiring resistance in every pixel. For electrically connecting the auxiliary wiring and the upper transparent electrode, it is necessary to remove all the organic layers constituting the organic light emitting element at contact hole portions. Accordingly, for the entire organic layer, a transport layer as well as the light emitting layer has also to be formed by using a precision mask, which results in a problem of lowering the mass productivity.